Offset stinger for arc lamp

ABSTRACT

In a gas-filled lamp an arc is initiated by withdrawal of a stinger electrode from contact with a stationary cathode, with subsequent transfer of the arc to the gap between the cathode and a stationary anode. The anode is cylindrical; and the stinger is elongate and is movably mounted within and electrically insulated from the anode. The cathode is elongate, having a long axis parallel to but spaced apart from the long axis of the stinger. The stinger contacts the cathode at other than the tip of the cathode, thereby precluding continual impacting of the stinger on the cathode tip with concomitant blunting of the cathode tip during the use cycle of the lamp. A magnetic core is affixed to the stinger, whereby the stinger can be moved by the energization of a solenoid. The magnetic core is apertured to provide a dashpot effect to damp the motion of the stinger.

United States Patent 1191 Stuart Dec. 3, 1974 OFFSET STINGER FOR ARC LAMP [75] Inventor: William R. Stuart, San Carlos, Calif. [73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Dec. 13, 1973 [21] Appl. No.: 424,399

52 u.s.c1 ..313/1s2,313/197,313/19s 51 Int. Cl. HOlj 61/00 [58] Field of Search 313/152, 197, 19s

[56] References Cited UNITED STATES PATENTS 6/1971 Beese 313/152 x Primary ExaminerJohn Kominski Assistant ExaminerDarwin R. Hostetter 'Attorney, Agent, or FirmStanley Z. Cole; Leon F.

Herbert; John J. Morrissey 57 ABSTRACT In a gas-filled lamp an arc is initiated by withdrawal of a stinger electrode from contact with a stationary cathode, with subsequent transfer of the arc to the gap between the cathode and a stationary anode. The anode is cylindrical; and the stinger is elongate and is movably mounted within and electrically insulated from the anode. The cathode is elongate, having a long axis parallel to but spaced apart from the long axis of the stinger. The stinger contacts the cathode at other than the tip of the cathode, thereby precluding continual impacting of the stinger on the cathode tip with concomitant blunting of the cathode tip during the use cycle of the lamp. A magnetic core is affixed to the stinger, whereby the stinger can be moved by the energization of a solenoid. The magnetic core is apertured to provide a dashpot effect to damp the motion of the stinger.

11 Claims, 3 Drawing Figures OFFSET STINGER FOR ARC LAMP BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is a further development with respect to gas-filled arc lamps. In particular, an improved electrode configuration is provided for an arc lamp in which the arc is initiated by the withdrawal of a stinger electrode from contact with a first stationary electrode with subsequent transfer of the arc to the gap between the first stationary electrode and a second stationary 'Chan application, an electric circuit is provided whereby a low-voltage powersupply can be used to start a gas-filled arc lamp wherein the arc is initiatedby the withdrawal of a stinger from contact with a stationarycathode, with subsequent transfer of the arc to the gap between the cathode and a stationary anode.

In a plasma discharge are between a cathode and an anode, the brightest portion of the arc occurs in a small region of the arc located-immediately adjacent the tip of the cathode. Most of the light from the arc originates in this small region, which is called the hot spot. In the arc lamp, in order to maximize the amount of light reflected out through the lamp window, it is necessary to fixedly position the tip of the cathode with respect to the reflector so that the hot spot remains constantly at the focus of the reflector during lamp operation. A technique for fixedly positioning the tip of the cathode with respect to the focus of the reflector is disclosed in US. Pat. No. 3,725,714, which issued on Apr. 3, I973 to Norman'C. Anderson and which is assigned to Varian. Associates. 5

In an'arc lamp of the type disclosed in the Chan application' mentioned above, it is desirable to mount the cathode with respect to the reflector so that the tip of the cathode will remain fixedly positioned'with respect to the focus of the reflector, not only during the manufacturing process but also throughout the expansion and contraction of the lamp components during the lamps use cycle. The mounting technique disclosed in the Anderson patent mentioned above assures that the i cathode itself 'willremain fixedly positioned with respect to the reflector during temperature cycling of the lamp. However, it has been found .that repeated impacting of the stinger on the tip of the cathode tends to blunt the cathode tip, thereby changing the location of the hot spot of the are relative to the focus of the reflector. v p

SUMMARY or THE INVENTION This invention p rovides'an electrode configuration for an arc lamp whereby the repeated impacting of a stinger electrode against a fixed electrode during the use cycle of the arc lamp will not blunt the tip of the fixed. electrode.

Accordingly, it is an object of this invention to provide an arc lamp in which the arc is initiated by the withdrawal of a stinger from contact with a fixed cathode, with subsequent transfer of the arc to the gap between the cathode and a fixed anode, wherein the stinger makes contact with the cathode at other than the tip of the cathode.

It is likewise an object of this invention to provide an arc lamp in which the electrodes comprise a cylindrical fixed anode, an elongate stinger movably mounted within and electrically insulated from the anode, and an elongate fixed cathode, wherein the long axis of the cathode is parallel to but spaced apart from the long axis of the stinger in such a way that the stinger does not come into contact with the tip of the cathode during the use cycle of the lamp.

It is a further object of this invention to provide an arc lamp wherein the movement of a stinger electrode in either direction between an extended position in contact with a fixed electrode and a retracted position away from contact with the same fixed electrode is damped by the dashpot effect of gas within the lamp passing through an aperture in a member affixed to the stinge'r.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, the arc lamp of this invention comprises a gas-filledhermetically sealed envelope I, typically containing xenon gas. An elongate metal cathode 2is fixedly mounted within the envelope. A metal anode 3 in the form of a hollow cylindrical structure is likewisefixedly mounted within the envelope, with the axis of the cylindrical anode structure 3 being parallel to yet spaced apartfrom the-long axis of the cathode 2. The cathode 2 and the anode 3 are mountedso that v a gap separates an apical end portion of one of these electrodes from an apical end portion of the other of these electrodes. A third electrode, which is an elongate metal stinger 4, is disposed coaxially within the hollow anode structure} and is electrically insulated therefrom by an insulating sleeve 5. The stinger 4 is slidably movable within the bore of the insulating sleeve 5 from a retracted position atwhich the tip of the stinger is completely withdrawn into the cylindrical anode structure 3 (as shown by solid lines) to an extended position at which the tip of the stinger contacts the cathode 2 at other than the tip of the cathode (as shown by broken lines). The stinger 4 is mechanically biased by a metal spring 8 toremain in the retracted position. Movement of the tip of the stinger out of the anode structure 3 into the gap between the anode and the cathode occurs when solenoid coils 9 are energized, which counteracts the mechanical bias of the spring 8.

The arc lamp also comprises a generally cylindrical ceramic side envelope wall member 12. A transparent window disc 13 is hermetically sealed to the front ,end of ceramic cylinder 12 by means of metal rings l4, l5

and 16 all brazed together. Ring 14 is metallically bonded to the ceramic cylinder 12, and ring 16 is metallically bonded to the window 13. Cathode 2 is bonded to metal support arms 17 which extend radially out through slots in a reflector 18 adjacent the front edge thereof to a metal support ring 19. Support arms 17 are brazed to support ring 19, which is brazed to ring 14, thereby forming a continuous electrical path from the cathode 2 to the outer metal ring 14 which cylindrically surrounds the front portion of the lamp, i.e., which encompasses all portions of the window-end of the lamp mounted forwardly of the insulating ceramic cylinder 12. The front end of the reflector 18 is also brazed to support ring 19, so that the reflector 18 maintains the same electrical potential as the cathode 2. Since the window-end of the lamp is the end that is exposed to the external environment, it is contemplated that the cathode 2, and therefore all components at the same electrical potential as the cathode, will be maintained at ground potential.

Anode 3 is brazed in a bore provided in massive metal base member 20, which serves to conduct heat away from the anode. Base member 20 is electrically insulated from the forwardly mounted members of the lamp by means of ceramic cylinder 12. A hermetic seal is provided between base member 20 and ceramic cylinder 12 by metal ring 21 which cylindrically surrounds adjacent portions of base member 20 and ceramic cylinder 12. Base member 20 is bonded to ring 21 by brazing, and ceramic cylinder 12 is bonded to ring 21 by any well-known ceramic-to-metal sealing technique applied to the outside rim portion of the ceramic cylinder 12 adjacent ring 21. Ring 21 also serves to provide an electrical lead path through base member 20 to the anode 3, whereby anode 3 can be maintained at an I electrical potential different from cathode 2. Within the hollow anode structure 3, insulating sleeve is metallically bonded thereto. Stinger 4 is slidably movable within the bore of insulating sleeve 5 and is prevented by sleeve 5 from contacting the anode 3, so that the anode and the stinger are electrically isolated from each other.

Affixed to the rear of base member 20, surrounding the bore in which anode 3 is brazed, is a metal cylinder 26. Hermetically bonded to metal cylinder 26 is a ceramic insulating cylinder 27, and hermetically bonded to ceramic insulating cylinder 27 in a metal cylinder 28. Cylinders 26, 27 and 28 are coaxial with the long axis of stinger 4, and serve to house the elongate stinger. Metal cylinder 28, at its end remote from ceramic cylinder 27, terminates with a metal end plug 29 and with a metal pinch-off tubulation 30. The inner wallsof cylinders 26, 27 and 28 and of end plug 29 and pinched off tubulation 30 comprise portions of the hermetically sealed envelope 1, with gas communication from the window side to the solenoid side of base member being provided by a bore 31 through base member 20. Ceramic cylinder 27 serves to electrically isolate metal cylinder 28 from metal cylinder 26 and therefore from anode 4. The inside diameter of ceramic cylinder 27 is smaller than the inside diameter of the adjoining metal cylinder 28, so that the end of ceramic cylinder 27 remote from metal cylinder 26 presents to the region within metal cylinder 28 a wall-like barrier 33 having a central apertured portion through which the stinger 4 can pass. The bore 31 in base member 20 and the recess 32 in the inner surface of end plug 29 permit evacuation of the lamp and subsequent filling thereof with a desired ionizable gas through tubulation 30 before it is pinched off.

Solenoid coils 9 surround cylinder 28. Attached to the end of stinger 4 remote from the cathode 2 is a magnetic core 25 as of iron, which fits slidingly within cylinder 28. The metal spring 8 is disposed within cylinder 28 between the iron core 25 and the apertured wall 33 which is presented by cylinder 27. Spring 8 is biased to keep the iron core 25 in contact with the end plug 29. When the solenoid coils 9 are energized through lead lines 46 and 47, the iron core 25, which is the armature of the solenoid, is caused to move forwardly against the spring bias. Stinger 4, which is affixed to the iron core 25, is thereby carried forward to the position shown by broken lines in FIG. 1 into contact with cathode 2. An electrical path is maintained from lead line 44 to the stinger 4 by the sliding contact which exists between the iron core 25 and the inner wall of metal cylinder 28, and by the contact of spring 8 with both iron core 25 and metal cylinder 28. Lead line 44 makes electrical contact with metal end plug 29 which is brazed to metal cylinder 28. To initiate the arc, the solenoid coils 9 are first energized to bring the stinger 4 into contact with the cathode 2. A power supply is also connected to the cathode 2, the anode 3 and the stinger 4. (One suitable circuit for connecting a power supply to these electrodes is disclosed in the above-mentioned Pat. application Ser. No. 337,234). Then, the solenoid coils 9 are deenergized. The mechanical bias of spring 8 thereupon causes the stinger 4 to be retracted from contact with cathode 2 to a position, as shown by solid lines in FIG. 1, at which the tip of the stinger is within the region enclosed by the cylindrical anode structure 3. As the tip of the stinger separates from contact with the cathode 2, a difference of electric potential develops between the stinger 4 and the cathode 2, which is sufficient to ionize the gas in the gap therebetween. As the stinger 4 withdraws into the anode structure, the arc in the gap between the stinger 4 and the cathode 2 initially follows the stinger 4. However, as the tip of stinger 4 moves into the region enclosed by the anode structure 3, the arc will jump to the apex of the anode 3 as will be discussed more fully below. After the arc has been created, it can thereafter be maintained at a typical current value of about 50 amperes with a typical difference of electrical potential of about 20 volts between the anode 3 and the cathode 2.

The principal feature of this invention is that the electrodes are configured so that the stinger 4 does not impact the cathode 2 at the cathode tip when solenoid 9 is energized. In a plasma discharge arc between a cathode and an anode, the brightest portion of the arc occurs in a small region of the are located immediately adjacent the tip of the cathode. Most of the light from the arc originals in this small region, which is called the hot spot. In an arc lamp, in order to maximize the amount of light reflected out through the lamp window, it is necessary to fixedly position the tip of the cathode with respect to the reflector so that the hot spot remains constantly at the focus of the reflector throughout the use cycle of the lamp. in the arc lamps known to the prior art, wherein the arc is initiated by withdrawing a stinger from contact with a stationary cathode, the stinger continually impacts the tip of the cathode throughout the use cycle of the lamp. in other words, every time a prior-art lamp is to be started, the

stinger is driven by a solenoid into collision with the tip of the stationary cathode. Such continual impacting of the stinger on the cathode tip results in blunting of the cathode tip with a resulting change in the location of the cathode tip relative to the reflector. Such deformation of the cathode tip is especially severe when the lamp is to be turned on, off, and on again within a short period of time. After the arc is initiated, the tip of the cathode becomes white hot. If the lamp is turned off, and then the arc is reinitiated before the cathode has had a chance to cool off, the momentum transfer caused by the impacting of the stinger 4 on the hot cathode tip will readily cause deformation of the cathode tip. It should be noted that it is important for the cathode tip to terminate in as sharp a point as possible in order that the position of the hot spot can be defined dency of the arc to roam around the edge of the anode was an instability that increased the likelihood that the arc might jump to some other metal component of the lamp at anode potential. The most stable arc, i.e., the arc least likely to jump to other metal comas precisely as possible. The surface of the reflector is be determined by optical techniques. Since the hot spot may for practical purposes be taken to lie immediately adjacent the cathode tip, it is desirable to locate the cathode tip precisely at a focus of the reflector, or at the focus of a particular portion of the reflector which comprises a segment of a particular surface of revolution. If, for example, the reflector surface comprises a segment of an ellipsoid, it may be desirable tolocate the cathode tip such that the brightest spot in the arc gap, i.e., the hot spot, will be positioned precisely at the internal focus' of that ellipsoid so that the maximum amount of light will be reflected out through the arc lamp window.

To achieve the object of this invention which is to preclude impacting of the stinger on the apex of the stationary cathode, the electrodes are configured so that the axis of the stinger is offset from the axis of the cathode whereby the stinger will make contact with the ponents of the lamp, would be an arc that is maintained in a gap between only two points, which two points are closer. together than any other two points between which the arc discharge is possible. Therefore, according to this invention, the anode 3 is designed to have an apex 53 that is closer than any other portion of the anode 3 to the tip of the cathode 2. As can be seen in FIG. 1, the anode 3 has a flat end face 51 perpendicular to the axis of the cathode 2, with a spike-like projection 52 extending from the end face 5.] toward the cathode 2. This projection 52 is shown in frontal view in FIG. 2 and in side elevation in FIG. 3. The apex 53 of pro jection 52 is disposed coaxially with the tip of the cathode 2 on the axis of cathode 2. Thisdisposition of apex 53 over the prior art resides in the provision of channel 60 cathode at otherthan the cathode tip. As is shown in FIG. v1, the stinger 4 and anode 3 are coaxial with each other; and the coaxes of the stinger 4 and of the anode 3 is parallel to but spaced apart from the axis of the cathode'2. The stinger 4 will therefore impact the cathode 2 at a place on the cathode 2 where the PSI loading, i.e., the momentum transfer in a given time interval per unit area expressed inpounds per square inch, will be much slower than at the tip of the cathode. As can be seen in FIG. 1, the tip of the stinger 4 is shaped to present a flat contact interface with the area of the cathode 2 which it impacts. This feature results in the lowest possible PSlloading on the cathode 4, and would not have been possible if the stinger were requiredto impact the cathode 4 at the cathode tip',

A further improvement which this invention provides 7 over the prior, art lies in the design of the anode structure 3. According to the prior art, a suitable anode structure would be a hollow cylindrical metal structure completely symmetrical about its axis. In a prior art lamp, as the stinger was withdrawn into the anode structure, the arc in the gap between the stinger and the cathode would jump from the stinger to some place on the edge of the anode facing the tip of the cathode the cathode tip. Since the anode structure in the'prior art was'symmetrical about its axis, all points on the anode edge facing the tip of the cathode were (in prinas soon as the gap distance between the tip of the through iron core 25, whereby a dashpot effect is created as the iron core 25 moves within the housing cylinder 28. Gas within the envelope 1 passes through channel as the stinger 4 is moved in either direction. The

resulting dashpot effect serves to damp the motionof the stinger, thereby preventing unnecessarily jarring the iron core 25on the end plug 29. Y

Since changes could be made in particular details of the embodiment of the invention disclosed hereing without departing from the scope of the inventiomit is intended that the above description and accompanying drawing be interpreted as illustrative only and not as limiting.

What is claimed is:

1. An arc lamp comprising a sealed envelope containing an ionizable gas, first and second electrodes mounted in fixed positions in said envelope, each fixed electrode having an apical end portion spaced apart from an apical end portion of the other fixed electrode to form .an arc gap therebetween, a stinger electrode movably mounted in said envelope, means for moving said stinger between a first position in contact with a surface portion of said flrst'flxedelectrode to a second position away from contact with said first fixed electrode, said surface portion of said first fixed-electrode being located at other than the apex of the apical end portion thereof.

2. The are lamp of claim 1 wherein said first fixed and electrically insulated from said second fixed electrode.

3. The arc lamp of claim 2 wherein said cylindrical second fixed electrode comprises a flat end face perpendicular to the cylindrical axis thereof, said apical end portionthereof being a projection from said end face.

4. The are lamp of claim 1 wherein a portion of said envelope is generally cylindrical, said stinger is elongate within said cylindrical portion of said envelope, a magnetic core is affixed to said stinger and is diposed to move in sliding contact with the interior surface of said cylindrical portion of said envelope, and a spring biased against said sliding movement of said magnetic core is mounted within said cylindrical portion of said envelope.

5. The are lamp of claim 4 wherein said magnetic core is apertured so that said movement of said magnetic core provides a dashpot effect with respect ot said gas, whereby said movement is clamped.

6. The arclamp of claim 5 wherein said sliding movement of said apertured magnetic core permits the passage of a portion of said gas along a path through said magnetic core parallel to the long axis of said stinger.

7. The arc lamp of claim 2 wherein said stinger is elongate and is mounted so that the long axis thereof substantially coincides with the axis of said cylindrical second fixed electrode, said long axis of said stinger being parallel to and spaced apart from the long axis of said first fixed electrode.

8. The are lamp of claim 2 wherein the apex of the apical end portion of said second fixed electrode lies on the long axis of said first fixed electrode.

9. The are lamp of claim 4 wherein said spring is disposed intermediate said magnetic core and said second fixed electrode.

10. The are lamp of claim 1 wherein said envelope further contains a reflector, said reflector comprising a concave reflective surface disposed generally symmetrically about an axis coincident with said are gap, a portion of said reflective surface being a segment of an ellipsoid, a focus of said ellipsoid being located within said are gap.

11. The are lamp of claim 10 wherein said first fixed electrode is a cathode and said focus of said ellipsoid is located within said are gap immediately adjacent the apex of said cathode. 

1. An arc lamp comprising a sealed envelope containing an ionizable gas, first and second electrodes mounted in fixed positions in said envelope, each fixed electrode having an apical end portion spaced apart from an apical end portion of the other fixed electrode to form an arc gap therebetween, a stinger electrode movably mounted in said envelope, means for moving said stinger between a first position in contact with a surface portion of said first fixed electrode to a second position away from contact with said first fixed electrode, said surface portion of said first fixed electrode being located at other than the apex of the apical end portion thereof.
 2. The arc lamp of claim 1 wherein said first fixed electrode is elongate, said second fixed electrode is cylindrical, and said stinger is movably mounted within and electrically insulated from said second fixed electrode.
 3. The arc lamp of claim 2 wherein said cylindrical second fixed electrode comprises a flat end face perpendicular to the cylindrical axis thereof, said apical end portion thereof being a projection from said end face.
 4. The arc lamp of claim 1 wherein a portion of said envelope is generally cylindrical, said stinger is elongate within said cylindrical portion of said envelope, a magnetic core is affixed to said stinger and is diposed to move in sliding contact with the interior surface of said cylindrical portion of said envelope, and a spring biased against said sliding movement of said magnetic core is mounted within said cylindrical portion of said envelope.
 5. The arc lamp of claim 4 wherein said magnetic core is apertured so that said movement of said magnetic core provides a dashpot effect with respect ot said gas, whereby said movement is damped.
 6. The arc lamp of claim 5 wherein said sliding movement of said apertured magnetic core permits the passage of a portion of said gas along a path through said magnetic core parallel to the long axis of said stinger.
 7. The arc lamp of claim 2 wherein said stinger is elongate and is mounted so that the long axis thereof substantially coincides with the axis of said cylindrical second fixed electrode, said long axis of said stinger being parallel to and spaced apart from the long axis of said first fixed electrode.
 8. The arc lamp of claim 2 wherein the apex of the apical end portion of said second fixed electrode lies on the long axis of said first fixed electrode.
 9. The arc lamp of claim 4 wherein said spring is disposed intermediate said magnetic core and said second fixed electrode.
 10. The arc lamp of claim 1 wherein said envelope further contains a reflector, said reflector comprising a Concave reflective surface disposed generally symmetrically about an axis coincident with said arc gap, a portion of said reflective surface being a segment of an ellipsoid, a focus of said ellipsoid being located within said arc gap.
 11. The arc lamp of claim 10 wherein said first fixed electrode is a cathode and said focus of said ellipsoid is located within said arc gap immediately adjacent the apex of said cathode. 